1. Field of the Invention
The present invention relates to a ceramic heater and a gas sensor including the same.
2. Description of the Related Art
A gas sensor has been known which is attached to an exhaust pipe of vehicles, for example, and detects specific gas components contained in a detected gas such as an exhaust gas. A specific form of the gas sensor, for example, includes a detecting element detecting specific gas components in the detected gas and extending in an axial direction, and a ceramic heater heating the detecting element (for example, refer to JP-A-2010-120840).
In JP-A-2010-120840 and JP-A-2011-247657, the ceramic heater has a bar shape extending in an axial direction and includes a ceramic base body having a heater element in its interior, an electrode pad disposed on a surface of the ceramic base body and conductive with the heater element, a heater terminal member electrically connected to an external circuit, and a brazing material portion (bonding portion) bonding a terminal connection portion of the heater terminal member and the electrode pad.
In addition, in JP-A-2010-120840 and JP-A-2011-247657, the gas sensor includes an inner terminal member (contact terminal member) which is electrically connected to an inner electrode disposed in the detecting element and has a negative potential relative to the electrode pad. The contact terminal member contacts and holds the ceramic base body at a front end side thereof in the axial direction away from the electrode pad.
3. Problems to be Solved by the Invention
In the gas sensor disclosed in JP-A-2010-120840, the brazing material portion reaches the front end edge of the electrode pad. Thus, if atmospheric moisture deposits on the surface of the brazing material portion and the ceramic base body, migration may occur on the surface of the ceramic base body. In this case, metal contained in the brazing material is caused to move from the brazing material portion to the contact terminal member (portion coming into contact with the ceramic base body). As a result, a short circuit may arise between the brazing material portion and the contact terminal member.
In addition, in the gas sensor disclosed in JP-A-2011-247657, the terminal connection portion of the heater terminal member has a plate shape extending in the axial direction having a constant width dimension, and the front end edge thereof (front end edge of the terminal) has a linear shape (straight line orthogonal to the axis). In the case of using the terminal connection portion having this form, in a step of bonding the electrode pad and the terminal connection portion using the brazing material, the brazing material is wetly spread from the front end edge of the terminal to the front end side in the axial direction. However, a degree of wet spreading at one side is sometimes larger than at the other side, between a first pad side edge of the electrode pad (one of two pad side edges extending parallel to the axial direction) and a second pad side edge (the other of the above-described pad side edges). Consequently, the brazing material is likely to reach the front end edge of the electrode pad at one side. Therefore, as described above, migration may occur in which metal contained in the brazing material is caused to move and thereby a short circuit may arise between the brazing material portion and the contact terminal member.
In particular, when using a brazing material containing Ag, migration is likely to occur between the brazing material portion and the contact terminal member.